Technologies available for the rate quantification of gas(es) emitted from a point source such as a smokestack or a leaking pipeline are numerous and well understood. These techniques include use of rotameters or hot-wire anemometers to measure the velocity of gases escaping from a hole of known size. Also included are more high tech devices like a Hi-Flow sampler which is an instrument that combines a hydrocarbon detector with a flow meter and vacuum system.
Measurement of emission flux from larger, more complex non-point sources of emissions may be accomplished by numerical calculations within the facility (for example, calculations based on amount of material lost), by direct point-sampling of the gas plume downwind of the source, or (most recently) by use of path integrated optical remote sensing. Non-point-source emissions of gases like methane and other volatile organic carbons (VOCs) are numerous and variable and include refineries, industrial complexes, sewage systems, tank farms, landfills, agricultural sites, coal mines, oil and gas exploration and production sites and pipeline networks. Measuring the emissions from these types of sites is made more difficult by the fact that exact sources of emission are not always known.
There are several optical technologies available to quantify the gases emitted from non-point sources. These optical technologies include Open-Path Fourier Transform InfraRed Spectroscopy (OP-FTIR), Ultra-Violet Differential Optical Absorption Spectroscopy (UV-DOAS), Tunable Diode Laser Spectroscopy (TDLAS), and Path Integrated Differential Absorption Lidar (PI-DIAL). These optical remote sensing technologies are all ground based, active optical instruments which pass light through a plume of gas and measure a path-integrated concentration of gas in the plume by detecting changes in light passing through the gas plume. Further, these technologies rely on one or more retro-reflectors or separate light sources and detectors on either side of the gas plume.
Measurement approaches include horizontal plume mapping, vertical plume mapping and one-dimensional mapping downwind of a plume. Another measurement technique is Solar Occultation Flux (SOF) which uses IR and the sun as an optical source from a fixed or moving ground-based platform.
Many of the conventional techniques for measurement of gaseous emission are ground-based and require long term (several days) access to sites and unobstructed optical paths down-wind of the emission source(s). Remote or hard to access sites are difficult and expensive to measure. In addition, measurements are time consuming and take days or weeks to complete as an operator waits for the right wind conditions to direct a plume to a designated optical measurement path.
Because shifts in wind speed and direction result in an ever-changing plume of gas from a site, combining instrument readings from different optical paths over a period of time results in inaccurate flux calculations. Instruments like the SOF, which rely on the sun as an illumination source, require low cloud cover and high sun angle to produce sufficient light for usable measurements. In addition, using the SOF requires access to the site and a drivable road some distance downwind of the gas source.
These techniques only allow a partial optical view of ground-hugging plumes, since the instruments are mounted on the tops or sides of vehicles and cannot make measurements fully extending to ground level. Further, these techniques only allow a near instantaneous (a few seconds at most) snapshot of an entire plume cross section by using a single non-average wind speed and direction when calculating flux.
Accurate wind measurement is a major source of error for these techniques because the wind is constantly shifting. As a result, pre-positioning retro-reflectors and optical detectors to provide usable flux calculation is largely guesswork and frequently requires subsequent deployment of more ground equipment. These techniques also require access of a truck or vehicle to the site, which typically has to be shipped in from overseas. Because of their complexity and time consumption, the cost of using these technologies is high. Only a handful of sites may be measured a year.
Optical remote sensing techniques, on the other hand, are more efficient and require less time to detect gas plumes, by using instrumentation mounted in a fixed wing aircraft that flies hundreds of feet above the gas plume. An example of such instrumentation is the DIAL (differential absorption LIDAR) system, also referred to herein as the ANGEL system. The ANGEL system is described in U.S. Pat. No. 6,822,742, which is incorporated herein by reference in its entirety.
Briefly, the ANGEL system includes a sensor for remote quantitative detection of fluid leaks from a natural gas or oil pipeline by use of an airborne platform. The system includes a laser light source for illuminating an area of target gases and background. The target gases may be characterized by one or more absorption wavelengths (also referred to as on-line). The background may be characterized by a non-absorbable wavelength (also referred to as off-line) that is different from the target gases.
For example, the ANGEL system may use a 3-line tunable DIAL laser system for measuring the concentration path-lengths (CPL) of two selected target gases. When the airborne platform reaches a target location, laser beams are automatically pointed to the target location for scanning the surrounding regions. The returned laser beams are analyzed to develop two-dimensional gas-maps or images of gases, such as methane and ethane, in units of CPL.
In a 2-line tunable DIAL laser system, for example, two single wavelength, laser pulses are transmitted. One laser pulse of a specific wavelength is chosen which is absorbed by the gas of interest, and the other laser pulse, chosen at a different wavelength, is not absorbed. The energy reflected back to the sensor for both wavelengths is measured to generate an estimate of the target CPL. The energy reflected back to the sensor is described by the following relationship:
      E    ∝                            E          T                ⁢                  ρ          π                ⁢                  exp          ⁡                      [                                          -                2                            ⁢                              (                                                      CL                    p                                    +                                                            C                      bg                                        ⁢                    R                                                  )                            ⁢                              σ                ⁡                                  (                  λ                  )                                                      ]                                      R        2              ,                where ET is the transmitted energy,        ρπ is the surface reflectance,        CLp (same as CPL) is the concentration-length product of the gas,        Cbg is the background concentration of the gas,        R is the range to the surface, and        σ(λ) is the absorption cross-section of the gas as a function of wavelength.The target CPL is calculated in units of ppm-m (parts per million×meter).        
As will be described, the present invention relates to determining the emission rate, or flux of a gas plume emanating from a source, by using an aircraft that houses both the differential absorption LIDAR (DIAL) system, or the ANGEL system, and an airborne wind measurement system. The present invention improves on the accuracy of the emission rate by scaling the airborne wind measurement from the aircraft's flying altitude to the gas plume's near ground altitude.